The safe and efficient collection and treatment of wastewater is vital to communal health. However, municipal wastewater collection and treatment systems have produced flow fluctuations and low wastewater contaminant concentrations.
The biological wastewater treatment processes and facilities based on the activated sludge method consist of three major components: aeration tanks or zones, secondary sedimentation phase tanks or zones, and return sludge systems. In a conventional process modifications all three components have been physically separated, each from the others. In order to maximize the treatment system capacity and efficiency, the activated sludge concentration measured by Mixed Liquor Suspended Solids (“MLSS”) should be maintained as high as possible. However, the suspended solids sedimentation efficiency has been limited by the hydraulic conditions in the secondary sedimentation tanks. Most frequently, the gravity settling conditions have allowed maintaining the MLSS no higher than about 3,000-3,500 mg/L (ppm), which is below a level desired for optimum capacity and efficiency.
In the combined modifications herein for clarifying the sludge, the aeration tanks are (1) hydraulically connected with the phase separation for clarifying sludge and (2) the recirculation of the return sludge is provided by the intensity of aeration in the aeration zone causing liquid inflow from the clarification phase zone into the aeration zone. In this case, the MLSS can be increased to about 5,000-5,500 mg/L or more, thus increasing the capacity and efficiency of the treatment facility. Thus, the combined activated sludge treatment processes and facilities are more efficient than the conventional systems.
A goal of the present invention is to develop a cost-effective and efficient method and a combined unit that would maximize the MLSS in the activated sludge return flow, thus minimizing the intensity of aeration necessary to provide recirculation between the aeration and phase clarification zones.
The following is a brief description of the combined process-apparatus units of the prior art.
U.S. Pat. No. 6,159,365 to Kigel et al, issued Dec. 12, 2000, describes a method and apparatus for treating contaminated water, in particular wastewater, in a packaged modular treatment unit which is considered a fluidized bed reactor. The increase in the MLSS is provided by laminarization cartridges installed in the clarification zones as well as by a depressed bottom compartment in the clarification zone. Although the unit is desirable as it provides maintaining the MLSS up to about 7,000 mg/L this method and apparatus, nevertheless, has a significant drawback of the present invention compared to the currently described method and apparatus in that the necessity of increased aeration intensity for return sludge recirculation results in two-stage aeration for liquid inflow from the depressed bottom compartment.
U.S. Pat. No. 4,787,978 to Nicol, issued Nov. 29, 1988, describes a method for the purification of urban wastewater using highly concentrated activated sludge of a daily pollution sludge value (“BOD”) of about 10-15 kg BOD/m3/day, instead of an earlier used concentration of about 0.5-2.5 kg, thus increasing the efficiency of the use of the reaction volume. The increase in activated sludge concentration is achieved by separate thickening of the return sludge in a laminarization (inclined parallel plate) sedimentation tank. However, this method has the substantial disadvantage of being extremely complicated when compared with the fluidized bed reactors which combine the reaction and phase separation volumes in a single apparatus.
U.S. Pat. No. 4,707,252 to Durot et al, issued Nov. 17, 1987, describes a fluidized bed reactor for aerobic or non-aerobic biological treatment of wastewater which contains a fluidized bed of granular material. The treatment process is based on the three-phase gas-liquid-solids reactions and includes a device for removing air bubbles, a device for separation of the solid particles from the bacterial floc and from the discharged effluent, and a device for recirculation of the effluent which is a means for fluidization of the bed of granular material. Despite advantages it provides, it, nevertheless, has the drawback in that it is a very complex apparatus including numerous sections and compartments as well as moving parts which add to the cost of its maintenance and operation (“O&M”). Another drawback of the reactor is likely media channeling caused by a plug-flow fluidization regime. The channeled upward flow fluidization can result in inefficient use of the fluidized bed volume.
U.S. Pat. No. 4,869,815 to Bernard et al, issued Sep. 16, 1989, describes a fluidized bed reactor for biological treatment of liquids which contains granular media fluidized by the liquid to be treated. To avoid channeling, the fluidized media is supported by a flow distribution granular transfer material. The transfer material can compensate for the major disadvantage of the plug-flow upward fluidization of the fluidized granular media—potential media channeling. However, this again has the substantial drawback of being very complex.
U.S. Pat. No. 3,956,128 to Turner, issued May 1, 1976, describes an apparatus for treating industrial and domestic wastewater which is a cylindrical system divided into two concentric tanks, thus forming an intermediate space, considered to be an aeration chamber. The liquid is oxygenated under pressure and pumped through a spiral tube for transfer of soluble impurities into insoluble suspended particles by a conventional biochemical oxidation process. The admixture from the spiral tube which is released into the central zone where the activated sludge solids rise upward due to dissolved air, flotation are then collected on the surface and returned back to the process, and the separated effluent (subnatant) is removed from the reactor. The system requires pumps and devices for saturation of the liquid with the air as well as devices for collection and removal of floated scum, again making the apparatus highly complex. Another disadvantage of the reactor is that the spiral tube, which winds around outside the tank and is needed to make the system operate, is difficult and costly to construct.